Journal of Medical Physics
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Year : 2017  |  Volume : 42  |  Issue : 4  |  Page : 213-221

Grid block design based on monte carlo simulated dosimetry, the linear quadratic and Hug–Kellerer radiobiological models

1 Department of Medical Physics and Biomedical Engineering, Radiotherapy Oncology Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran
2 Department of Physics, University of Trieste and INFN Trieste, Italy
3 Department of Medical Physics and Biomedical Engineering, Research Center for Molecular and Cellular Imaging, Tehran University of Medical Sciences, Tehran, Iran
4 Educational Consultant, Las Vegas, Nevada, USA
5 Comprehensive Cancer Centers of Nevada, Las Vegas, Nevada, USA

Correspondence Address:
Hassan Ali Nedaie
Department of Medical Physics and Biomedical Engineering, Radiotherapy Oncology and Radiobiology Research Center, Cancer Institute, Tehran University of Medical Sciences, Keshavarz Blvd, Tehran
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jmp.JMP_38_17

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Purpose: The clinical efficacy of Grid therapy has been examined by several investigators. In this project, the hole diameter and hole spacing in Grid blocks were examined to determine the optimum parameters that give a therapeutic advantage. Methods: The evaluations were performed using Monte Carlo (MC) simulation and commonly used radiobiological models. The Geant4 MC code was used to simulate the dose distributions for 25 different Grid blocks with different hole diameters and center-to-center spacing. The therapeutic parameters of these blocks, namely, the therapeutic ratio (TR) and geometrical sparing factor (GSF) were calculated using two different radiobiological models, including the linear quadratic and Hug–Kellerer models. In addition, the ratio of the open to blocked area (ROTBA) is also used as a geometrical parameter for each block design. Comparisons of the TR, GSF, and ROTBA for all of the blocks were used to derive the parameters for an optimum Grid block with the maximum TR, minimum GSF, and optimal ROTBA. A sample of the optimum Grid block was fabricated at our institution. Dosimetric characteristics of this Grid block were measured using an ionization chamber in water phantom, Gafchromic film, and thermoluminescent dosimeters in Solid WaterTM phantom materials. Results: The results of these investigations indicated that Grid blocks with hole diameters between 1.00 and 1.25 cm and spacing of 1.7 or 1.8 cm have optimal therapeutic parameters (TR > 1.3 and GSF~0.90). The measured dosimetric characteristics of the optimum Grid blocks including dose profiles, percentage depth dose, dose output factor (cGy/MU), and valley-to-peak ratio were in good agreement (±5%) with the simulated data. Conclusion: In summary, using MC-based dosimetry, two radiobiological models, and previously published clinical data, we have introduced a method to design a Grid block with optimum therapeutic response. The simulated data were reproduced by experimental data.

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